docs: def Planck_frequency(nu, T):
      '''The Planck function B(nu,T):
      the flux density for blackbody radiation in frequency space
      nu is frequency in 1/s
      T is temperature in Kelvin
  
      Formula (3.1) from Raymond Pierrehumbert, "Principles of Planetary Climate"
      '''
      return 2*hPlanck*nu**3/c_light**2/(exp(hPlanck*nu/kBoltzmann/T)-1)